Apparatus for the manufacture of single crystals



1960 J- GOORlSS EN 2,956,863

APPARATUS FOR THE MANUFACTURE OF SINGLE CRYSTALS Filed Oct. 9. 1957INVENTOR JAN GOORISSEN AGENT United States Patent APPARATUS FOR THEMANUFACTURE OF SINGLE CRYSTALS Jan Goorissen, Eindhoven, Netherlands,assignor to North American Philips Company, Inc., New York, N .Y., acorporation of Delaware Filed Oct. 9, '1957, Ser. No. 689,197 Claimspriority, application Netherlands Nov. 28, 1956 2 Claims. (Cl. 23-473)This invention relates to an apparatus for the manufacture of singlecrystals from a substance, for instance a semiconductor such asgermanium or silicon, by drawmg a single crystal upwards from a melt ofthis substance in the form of a rod. In a further production stage thisrod may be divided into smaller bodies; for instance in the case ofsemiconductors these smaller bodies are used in semi-conductive devicessuch as transistors or crystal diodes.

Single crystals obtained by this known crystal-pulling technique have ahigh content of lattice defects, so called dislocations. In generalthese dislocations are undesirable, since they seriously interfere withthe physical properties, for example the conductivity, of a singlecrystal and may also adversely affect the further processing of a singlecrystal, for example diflusion or alloying.

It has been pointed out that dislocations are occasioned by thermalstresses in the freshly grown monocrystal rod when being at a hightemperature, which stresses are due to radiation at the surface andconcomitant un even cooling over the cross-sectional area of the rod. Inorder to counteract radiation from the surface as much as possible ithas been proposed, in the method termed zone leveling, to maintain thegrown monocrystal rod as a whole at a temperature only slightly lowerthan the melting temperature of the rod and, upon termination of thegrowing process, to cool the whole rod slowly and evenly.

In practice, the use of this known expedient in crystalpulling is notwell possible inter alia since in the crystalpulling process the rod issubjected to tensile stress and does not rest, as in zone-levelling, inan elongated horizontal boat during the growing process. Such a methodfurther suffers from a limitation in that it takes considerable time dueto the low temperature gradient at the surface of solidification, whichinvolves a slow growth. Moreover, such a method merely amounts toshifting the diflieulty to a later phase, that is to say the phase inwhich the Whole rod has to be cooled slowly and uniformly in order toprevent thermal stresses and dislocations originating therefrom.

The present invention has for its object to provide a simpler method anda device for carrying out this method, which constitutes an improvementupon the pulling method and yields good results without involving theaforesaid inconveniences.

It is based on the recognition that the heat radiated at the surface ofa freshly grown rod can be largely compensated by reflecting the thermalradiation from the surface of the melt, which is otherwise largelyradiated to the environment, to the surface of the rod, thus using it toadvantage.

According to the invention, said rod is at least partially drawn upwardsinside i.e. along the axis, of a reflecting screen which is locatedabove the melt and shaped as a truncated cone shell, the base of whichfaces the melt. Th angle subtended by the generatrix of the cone and theaxis of the rod is advantageously made smaller than 60", preferably 10to 30. The screen is preferably made from a material having a highreflection-coelficient, for example molybdenum, nickel or tungsten. Itmay sometimes be advantageous to use a screen made from a materialhaving a low reflection coefiicient, which satisfies other requirementsimposed by the circumstances and the inner wall of which is furnishedwith a reflective layer.

It has been found that when using the method according to the inventionthe etch-pit density of the crosssectional area of a pulled-upmonocrystal rod, which density is a measure of the dislocation densityof the rod, can be reduced to as low a value as one-tenth of that of amonocrystal rod made by means of the conventional pulling-method.

In order that the invention may be readily carried into effect, anexample will now be described in detail with reference to theaccompanying drawing, which represents schematically in cross-sectionalview that part of the crystal pulling apparatus according to theinvention, which is relevant thereto.

A graphite crucible 1 constains a germanium melt 2 and is heated byinduced high-frequency current by means of a high frequency coil 3. Theupper edge of the crucible 1 carries a quartz ring 4 on which is resteda reflecting molybdenum screen 5 in the form of a truncated cone shell,the imaginary base '6 of which faces the melt 2. The monocrystal rod 7is partially pulled up inside and more particularly in axial sense ofthe reflecting screen. The screen is approximately 8 centimetres highand the angle a subtended by the generatrix of the cone and the axis ofthe rod is approximately 20. The highfrequency coil 3 also partiallysurrounds the reflecting screen 5 so that this screen contributes, byradiation, to the reflected radiation. It has been found that thisexpedient means another appreciable improvement. The reflecting screenis provided with a number of holes 8 which permit inspection of thegrowing process inside the crucible. The crucible is separated from theenw'ronment by means of a quartz tube 9.

What is claimed is:

1. Crystal-pulling apparatus comprising a refractory vessel with asubstantially open top for holding a quan tity of molten semi-conductivematerial, a hollow, truncated, conical, conductive reflecting memberhaving open ends and mounted on and over the vessel with its wide endadjacent the vessel and with its inner reflecting surface exposed to thevessel interior, a coil surrounding the vessel for heating the vesselinterior by means of high-frequency currents, and means for drawing agrowing crystal upwards from the melt in the vessel and.

through the interior of the reflecting member along its axis, wherebythermal radiation from the melt surface is reflected onto the growncrystal thereby compensating radiation losses from the crystal and thusimproving its quality.

2. Apparatus as set forth in claim 1 wherein the coil also surrounds alower portion of the reflecting member and generates additional heattherein by high-frequency currents, and the angle subtended by the coneaxis and its generatrix is between 10 and 30.

References Cited in the file of this patent UNITED STATES PATENTS2,291,083 Jung July 28, 1942 2,686,212 Horn Aug. 10, 1954 2,753,280Moore July 3, 1956 2,809,136 Mortimer Oct. 8, 1957 2,839,436 CornelisonJune 17, 1958

1. CRYSTAL-PULLING APPARATUS COMPRISING A REFRACTORY VESSEL WITH ASUBSTANTIALLY OPEN TOP FOR HOLDING A QUANTITY OF MOLTEN SEMI-CONDUCTIVEMATERIAL, A HOLLOW, TRUNCATED, CONICAL, CONDUCTIVE REFLECTING MEMBERHAVING OPEN ENDS AND MOUNTED ON AND OVER THE VESSEL WITH ITS WIDE ENDADJACENT THE VESSEL AND WITH ITS INNER REFLECTING SURFACE EXPOSED TO THEVESSEL INTERIOR, A COIL SURROUNDING THE VESSEL FOR HEATING THE VESSELINTERIOR BY MEANS OF HIGH-FREQUENCY CURRENTS, AND MEANS FOR DRAWING AGROWING CRYSTAL UPWARDS FROM THE MELT IN THE VESSEL AND THORUGH THEINTERIOR OF THE REFLECTING MEMBER ALONG ITS AXIS, WHEREBY THERMALRADIATION FROM THE MELT SURFACE IS REFLECTED ONTO THE GROWN CRYSTALTHEREBY COMPENSATING RADIATION LOSSES FROM THE CRYSTAL AND THUSIMPROVING ITS QUALITY.